The proposed research centers on an investigation of gene transcription during oogenesis, the accumulation of gene transcripts in the mature egg and the possible role of the gene transcripts accumulated during oogenesis for the subsequent development and differentiation of the embryo. The developmental biology of Urechis caupo, the organism to be used in the proposed study, may be uniquely suitable for a study of this nature. There is no organized ovary in Urechis and the developing oocytes can easily be obtained in large quantities and separated according to stages of maturation. The mature eggs are readily available, easily fertilized and the embryos develop with excellent synchrony. Thus specific stages of development from early oogenesis through the trochophore larva stage of embyrogenesis are available and will be studied to characterize the quantity and quality of the gene transcripts present in the cells at each stage of development as well as the gene transcripts that are synthesized at each stage of development. The basic approach involves utilization of the RNA:DNA hybridization technique performed in solution under conditions that fulfill the kinetic requirements for hybrid formation with unique sequence DNA. The RNA:DNA hybridization studies will be performed to determine the genetic diversity of RNA present in the cells at various stages of development, the stage(s) of oogenesis when RNA present in the mature egg is synthesized and accumulated, the fraction of the RNA present in the mature egg that is stable and present in cells at subsequent stages of embryogenesis and the cellular distribution of RNA transcribed from unique DNA sequences and reiterated DNA sequences in eggs and embryos.